public $verifyCode;
['verifyCode', 'captcha'],
            ['verifyCode', 'required'],


<?= $form->field($model, 'verifyCode')->widget(Captcha::className(), [
                'template' => '<div class="row"><div class="col-lg-4">{image}</div><div class="col-lg-6">{input}</div></div>',
            ]) ?>
            <div style="color:#999;margin:1em 0">
                <?= Yii::t('app', 'Click on the image to get a new verification code.') ?>
            </div>

First, for SignUp Model.

Second, for signup view.

Yii2 signup captcha

php

PHP